Should we talk about speeds too? Something that's listed as running at 167 (like RAM) runs at 166.6 repeating and when it's DDR, it's 333, which is 333.3 repeating.
When Intel released the 486DX2 there wasn't any mention of fractions. It ran at 33 and 66 MHz. Then, when the DX4 arrived, things didn't really make sense because the external speed was still 33 MHz but the internal speed was nearly 100 MHz. Obviously, it should have been called DX3 instead of DX4.
Hard drive manufacturers can't really tell you how much less a drive will hold after formatting because it could be formatted with a lot of different file systems and none are going to take exactly the same amount of room for overhead. Even between FAT16, FAT32, HPFS, and NTFS, there are differences.
Remember when HFS+ arrived? Suddenly, the smaller cluster size allowed more available space when you had many small files such as text files.